My mother and I have/had a very close relationship that did cross the boundaries of parent/child to friends and tbh it really hasn't done me any favours at all.
Looking back I needed a parent but actually my mother treated me as a friend by leaning on me emotionally and over sharing every aspect of her life. I have a lot of issues now (too many to go into tbh!) that are down in part to the actually unhealthy relationship we had. ( there were other issues aswell which is a whole other thread)
You can view having a cool 'bff' relationship with your kids as great for you but is it really that great for your kids?
It's so important to have a parent in your life that can give you consistent boundaries and show you right from wrong which doesn't really work if you are having the cool best friend relationship.
I think having a friendship can also make a child feel unsafe, a friendship can change or be taken away if you do something the other doesn't like, you are constantly seeking reassurance that you haven't upset the parent and they will take away the 'friendship' whereas with a parent and child relationship it is clearer and more secure for the child knowing that the parent will love you unconditionally and will act as a secure, strong leader so the child doesn't have to lead themselves.
I cannot stress how important it is for a child to have an authoritative parental figure in their life for there stability and peace of mind.
My mother and I are very close still but I can see how a lot of my issues are down to not having a safe, secure childhood with proper parent/child boundaries. It was great being the cool kid with a cool parent but the reality and my future metal health was not quite so great.
I certainly wouldn't parent my own children in that way. I love them and am always there for them. We laugh and have fun and I like to think I am fairly easy going but I am a parent and ultimately I will keep them safe, secure and guide them/teach them. We are not on an equal playing field so cannot be equal friends they need me to be a parent.